A former Palmerston North mayoral candidate has been found guilty of assaulting a child - and the crucial piece of evidence was a video of the incident he recorded himself.

Ross Easton Barber, 50, was found guilty of the assault on his nephew, in which he teased and stood on the boy, after a trial before Judge Les Atkins in the Palmerston North District Court yesterday.

It is the fourth time Barber has been found guilty of assaulting a child.

Barber, who stood for mayor of Palmerston North at last year's election, filmed the assault, which took place on his piggery just outside the city.

On a date between October 2006 and October 2008, Ross's nephew - who was between 8 and 10 years old at the time - was at the piggery.

The two had an argument, and the boy became agitated.

Barber grabbed his video camera and filmed what happened next - something he later told Detective James Bugg was done to show the boy how bad his behaviour was.

The boy walked out of an office on the piggery and Barber followed.

He called him a "cry baby" and told him to "smile for the camera".

The boy tried to throw some things at Barber before walking onto the driveway, where he either tripped or was pushed over.

Barber then held the child down by putting one foot on his chest and holding one arm, as the boy cried and coughed.

In his police interview, Barber told Detective Bugg the boy was fine at that moment.

He said he had provoked his nephew to try to get him to talk instead of reacting violently, and he held the boy to stop him being violent or throwing things.

As the incident took place before the so-called anti-smacking law was passed, which removed the use of reasonable force as a defence, the new legislation did not apply to Barber's case.

"What the defendant was doing was behaving in a way which could only have further provoked, distressed and aggravated the child to almost the point where the child was unable to be reasonable," Judge Atkins ruled.

"It would not teach any child anything other than cruelty."

Judge Atkins convicted Barber of the charge and remanded him on bail until May for sentencing.